Crystal polystrene is a term which is applied to homopolymeric polystyrene containing no reinforcing polymeric material such as rubber. Crystal polystyrene is normally a clear, colorless, transparent resin, used for making injection molded articles and films. Another large application is in the manufacture of expanded thermoplastic polystyrene foams, in which the crystal polystyrene is mixed with a blowing agent, e.g. pentane or fluorocarbon compounds, under conditions causing vaporization of the blowing agent while mixed in the molten polystyrene.
The resistance of crystal polystyrene to oxidative degradation has generally been regarded as adequate in itself, so that stabilizer systems for it have not received extensive investigation. The art is replete with descriptions of stabilizers for impact polystyrenes. The problem of oxidative degradation is much more prevalent with impact polystyrene, however, because of the presence therein of the reinforcing rubber which contains carbon-carbon unsaturation. However, the stabilization of crystal polystyrene, which contains no unsaturated rubber, is not normally a problem.
There is, however, at least one specific area where oxidative degradation of polystyrene is a signficant problem. This is in relation to the production of thermoformed articles from extruded foam polystyrene sheet. During such a process, it has been found that oxidative degradation of polystyrene is likely to occur to an extent having an adverse effect on the physical properties of the thermoformed articles.